'Serena is an incredible champion'

The 32-year-old, who became the oldest women's player to clinch the top ranking when she returned to No 1 in February, has won more than $9 million in prize money in 2013 so far, the WTA said in a statement.

"Serena has proven time and time again throughout her career that she is an incredible champion, both on and off the court," WTA Chief Executive Stacey Allaster said in the statement.